In adults, the lungs have been shown to be involved in the regulation of many circulating vasoactive and "neural hormone" substances. The development of the metabolic processes required for this regulation is for the most part unknown. A major portion of the metabolic functions of adult lungs have been studied in isolated, perfused lung preparations. The co-investigator has developed a system for perfusing lungs isolated either from newborn or from previable, non-breathing fetal rabbits. These preparations will be used as a tool to study pulmonary metabolic functions in fetal and newborn lungs. These data will be compared and contrasted to work done previously by others in isolated perfused adult lung preparations. Our working hypothesis is that the ability of lungs to metabolize blood constituents parallels the development of the gas exchanging function. In any case, elucidation of the nature of pulmonary metabolic development will contribute to the management of critically ill children in whom inadequate pulmonary metabolism might result in imbalances normally not encountered in adults.